Method for Producing Polyesters and A Disk Ring Reactor for the Method

ABSTRACT

The invention relates to a method and a device for producing polyesters such as for example polyethylene terephthalate, polypropylene terephthalate and polybutylene terephthalate from precondensates. According to said method, vapors comprising precondensate components that are distributed therein in the form of an aerosol are guided through a polycondensation reactor in which precondensate components condense on the reactor wall and on a separator in an exit chamber of the reactor. The condensates are guided to the unstirred discharge sump and the upper layers of the discharge sump are continuously recirculated into the stirred reactor area, thereby subjecting them to reconversion and additional polycondensation.

The invention relates to a method for producing polyesters fromliquefied precondensates and to a disk-ring reactor with an unstirredoutlet chamber suited for this method.

Disk-ring reactors used for the continuous production of polyesters havebeen known for some time. These are typically cylindrical, horizontal,heated vessels with inlet and outlet connections for the precondensateand polycondensate on opposing ends of the disk-ring reactor. Thedisk-ring reactor comprises a plurality of elements rotating about ahorizontal axis, which elements mix the precondensate and produce alarge surface for outgassing the polycondensate when the viscousliquefied material adhering to these elements run down. Devices of thistype are described in the German patent applications 1 745 541, 1 720692, 2 100 615 and 2 114 080 as well as in the European patents andpatent applications 0 320 586, 0 719 582, 0 711 597 and 1 386 659.

The disadvantage with known disk-ring reactors is that polycondensateresidue deposits on the inside wall of the reactor, resulting not onlyin encrustation and fouling of the reactor, but additionally producing adiscolored product with undesirable inclusions if the product adheres tothe inside reactor wall for an extended period and is exposed to highreaction temperatures, which result in thermal damage to andcross-linking of the polycondensate. Non-filterable, gel-likecontamination interferes with the polymer processing operation and inthe end reduces the product quality.

The object is therefore to develop a novel method and an improveddisk-ring reactor, in which during the polycondensation operation aself-cleaning effect occurs that prevents the formation of encrustation,deposits and defective product on the inside reactor wall and thusguarantees a consistently high product quality.

The object of the invention is therefore a method for producingpolyesters, such as polyethylene, polypropylene and polybutyleneterephthalate from precondensates, according to which vapors comprisingprecondensate components distributed therein in the form of an aerosolare fed through a polycondensation reactor in which precondensatecomponents precipitate on the reactor wall and in an outlet chamber ofthe reactor on a separator. The precipitate is then conducted into theunstirred discharge sump and the upper layers of the discharge sump arecontinuously recirculated into the stirred reactor area and thussubjected to reconversion and further polycondensation.

It is preferred if a disk-ring reactor is used as the polycondensationreactor, in which in the interest of uniform residence times and acontrolled viscosity increase the reaction chamber in the sump isdivided into several compartments by separating plates, whichcompartments are connected to one another by means of outflow openings.A disk-ring reactor of this type according to the present invention forproducing polyesters comprises a heatable, cylindrical, horizontalvessel with at least one inlet for the melt and at least one outlet aswell as at least one vapor outlet above a melt outlet and an internalagitating system that is adapted to the vessel shape. This agitatingsystem includes vertically disposed annular disks that are attached bymeans of spokes to at least one common horizontal drive shaft. Backflowis achieved from the unstirred outlet chamber through at least oneopening of the partition into the upstream, stirred compartment. Sincethe contents of the outlet chamber are not moved by means of rotatingelements, reliable level measuring and a minimum product level arepossible; the discharge of the finished polycondensate is not disturbed.

In a disk-ring reactor of this type, the openings provided in the lastpartition are at least one product outflow opening (preferably oppositethe product outlet on the bottom on the side of the product sump raisedby the rotating annular disks, meaning the emersion side of theagitator) and at least one product backflow opening (on the oppositeimmersion side of the agitator), which is preferably configured as awall-side peripheral opening expanding steadily from the lowest point ofthe vessel to exclude stagnating sump zones.

Improved level control and level minimization are achieved by means of afurther overflow opening (delimiting the discharge sump) that isprovided radially inside the last annular disk. This reduces circulationflow between the last agitated compartment and the discharge chamber,preventing breakdown of the polymer as a result of excessive residencetimes in corners without flow and product discoloration.

Since due to the high viscosity the product adheres to the agitatorelements, a kidney-shaped profile in cross-section develops in the sumpthat is considerably higher in the direction of rotation than on theopposite side on which the agitator elements dip down into the sump. Dueto the raised product outflow opening on the exit side of the agitatorelement, the polymer gushes into the outlet chamber and then flowsthrough the wall-side opening on the agitator element entrance side backinto the stirred chamber, an immersed partial product flow leaving theoutlet chamber via the outlet nozzle (FIG. 1). The positions of theinlet and outlet openings in the last partition allow the flowdirection, flow intensity and the sump level to be controlled. Due tothe raised overflow opening, product is also conducted from the surfaceof the outlet chamber into the last stirred compartment, so that theprecondensate material dripping from the baffle and flowing off the backwall is fed to a proper reaction in the main polymer mass.

When using suitable process conditions, the design of the disk-ringreactor according to the present invention causes the precondensate toreach the vapor flow first as a result of the foaming of the reactionmass and the splashing of droplets from the agitators containing theproduct against the reactor wall (particularly when vapor bubbles burston the agitator surface), second as a result of the suspending andentraining of fine aggregates (droplets, foam lamellae) and produces thesubsequent precipitation of precondensate from the vapors on the reactorwalls. According to the invention, this process is increased whenguiding the vapors in a targeted fashion in the upper reactor part (whensufficiently high resistance against axial flow have been installed inthe inside reactor area) and when increasing the peripheral movement ofthe agitators in the reactor entrance region and the secondary flow(vortex cells) between the annular disks and the upper reactor wall.Since the gas outflow according to the invention is provided on theproduct discharge side of the disk-ring reactor, low-viscosityprecondensate components also reach the walls at the back of the reactorin the form of a film together with the droplet-containing aerosol-likevapors, loosening the otherwise tough films and making them flowablebefore thermal damage and residue formation can occur in the leakair-free reactor system (FIG. 2). This effectively counteracts anyencrustation or deposits of polyesters on the inside reactor wall.

The separator provided in the outlet chamber of the reactor prevents theexcessive discharge of entrained product in the exhaust gas flow, whichwould interfere with the condensation operation, and can be configuredas a baffle, a mist collector, a demister or in the form of anothersuitable flow deflection arrangement. A simple baffle with no dead spaceis particularly preferred for reasons of limited loss of pressure. Thisbaffle may have a rim or not. The vapor nozzle for the gas outflow mayhave a collar or not.

To make the self-cleaning effect according to the invention as effectiveas possible, the intrinsic viscosity of the precondensate should be nomore than 0.37 dl/g, preferably between 0.21 and 0.33 dl/g. The methodfor determining the intrinsic, viscosity is described, for example, inDE 101 58 793 A1, page 5, lines 41-42. For the desired, partialpreliminary separation of precondensate as a wall film, according to theinventive method the vapors are conducted along the ceiling wall,preferably in a sickle-shaped clearance between the reactor housing andthe agitators, which clearance is present due to the eccentricity of theagitator axis relative to the housing axis. The preferred flow developsin detail due to the compartmentalized or covered product chamberextending to the agitator shaft, the hydromechanical, kinematic closureof the inside agitator space by the polymer films flowing off the panelsand spokes, reinforced by an alternating offset of adjoining annulardisk agitators by about half a spoke sector angle, and optionally due tothe static or preferably rotating compartmenting of the vapor chamberaccording to the invention.

Furthermore, the method as described above is preferably implementedsuch that the outlet chamber has a stationary, controlled minimumproduct level.

The above-described disk-ring reactor can be further improved through aseries of special design characteristics.

To intensify the mixing process and the renewal of the surface, it maybe advantageous to use a second horizontal axis of rotation as anextension of the first axis of rotation to operate at a speed that isbetter adjusted to the viscosity and with reduced energy input. Sincethe product entrainment and the film formation on the agitator elementsdepend not only on the design of these elements, but particularly on theviscosity and the circumferential speed, it may additionally beadvantageous to modify the circumferential speed in accordance with theprogress of the reaction. Typically, the ratio of the viscosity uponentering the reactor to the viscosity upon exiting the reactor is about1:100 to 1:200.

When in a disk-ring reactor in addition to a first agitator shaft also asecond agitator shaft rotating at reduced speed is provided, this secondshaft can be guided with the same or greater eccentricity of the shaftaxis relative to the vessel axis in the exit region. Advantageously,product-lubricated bearings are provided to support and seat theagitator shafts inside the reactor. Normally, instead of two shaftseals, this way a single shaft seal is achieved, as well as a lower riskof leaking air affecting the quality.

To achieve uniformly good mixing of the polycondensate in the sump aswell as good film formation at the same speed in the cylindrical vessel,it is recommended to vary the spacings between in addition to the designof the agitator elements. This change of the spacing creates space, sothat the more viscous film product can be mixed in the sump and theenergy input remains limited due to the growing shear gap.

As a result of the second drive shaft, the rotational speed can bebetter adjusted to the viscosity and due to the reduced rotational speeda smaller spacing can be set between the agitator elements, resulting ina higher specific surface and making it possible to achieve a higherviscosity ratio of 1:400 to 1:1000. This second drive allows inparticular the production of highly viscous products and a maximizationof the specific surface, allowing the implementation of more powerfulapparatuses.

The intrinsic viscosity (IV) of different polymer products can thus beincreased to values of less than or equal to 1.35 dl/g, preferably inthe range of IV=0.5 dl/g to 1.05 dl/g with one reactor shaft and toIV=0.5 dl/g to 1.25 dl/g with two reactor shafts.

A preferred embodiment comprises a heatable, cylindrical, horizontalvessel with an inlet for the melt on one end and an outlet on the otherend as well as a vapor outlet in the downstream wall above the meltoutlet, so that the precondensate flow is guided uniformly, meaningundivided, through the reactor.

Particularly high throughput is achieved with a disk-ring reactor havinga reduced diameter when the precondensate flow is divided into twopartial flows that are conducted entirely or partially separatelythrough the reactor. For this purpose, two precondensate flows can befed on the cover of the reactor and the end product can be discharged inthe center. This configuration cuts the exposure to vapor in half forany given reactor diameter.

It is also possible, however, to achieve high throughput in a disk-ringreactor with limited diameter when the precondensate flow is fed in theregion of the reactor center, the flow is divided into two partial flowsand both partial flows are conducted toward each other to separateproduct outlets on the cover. This creates the additional possibility ofindividually controlling the viscosity of the two end products by meansof the rotational speed and by temperature control. The disadvantage isthat two gas outlets are required.

All characteristics according to the invention can be implemented foruniform as well as, with the appropriate adjustment, for dividedprecondensate flow guidance.

The configuration of the different agitator elements on a shaft by meansof spokes has the advantage of a simple, flexible and robust design thatis easy to assembly and maintain since the agitator elements can beremoved individually from the shaft. The cage-like designs described forexample in the European patent application 0 711 597 or in the Europeanpatent specification 0 719 582, on the other hand, are complex weldeddesigns that are difficult to maintain. There as well two shaft holesare provided, while in the design according to the invention with aninside bearing only one shaft hole per drive shaft is required. Thecentral hollow cylinder on the cage-like agitator is associated with thedisadvantage that undesirable longitudinal mixing may occur in the axialdirection due to entrainment of dripping polymer and foam development.If the shaft is instead mounted eccentrically in the housing, an uppersickle-shaped compartment is created through which the flow of gas canexit. In the case of higher viscosities, there is a tendency that thematerial flowing off the agitator elements accumulates on the shaft andlimits the polycondensation activity. For this reason, it is expedientto clean the shaft by means of special scrapers, particularly in theupper viscosity range. In the medium viscosity range, a crossbar closeto the shaft provided between individual agitator elements will suffice.Possible agitator elements are primarily annular disks or disk segmentsthat may be configured as solid or perforated disks. These disks can beprovided individually or in an assembly and can also be equipped withscoops.

The free segments formed between the spokes can alternately be coveredwith perforated sheet metal plates. Disk ring agitators with an evennumber of spokes can thus alternately comprise a regular free sector anda covered sector. When connecting, as is described according to theinvention, at least two of these annular disks in series with axiallyaligned spokes such that in the axial perspective free and coveredsectors alternately overlap, the axially flowing gas is effectivelydeflected, separating foam and entrained drops in the axial direction.Such deflection of the axial flow of gas results in an increase in thevertical flow component and the gas speed in the sickle-shapedcompartment. In the product sump at the same time a compartmentingeffect exists that prevents low-viscosity material from shooting throughuncontrolled. Instead of the perforated plates, it is also possible touse unperforated plates or wire mesh.

A transition to alternating spokes offset by half a spoke sector anglefor consecutive disk ring agitators, which transition is preferred asthe melt viscosity increases, serves not only the even blanketing of theinside agitator space, but also the improved mobility and mixing of theproduct sump. For increased axial conveyance of the product melt, it isadditionally possible to offset the spokes of consecutive agitators inthe axial direction in a screw-like manner, trailing by an angle of 0.4to 4 degrees.

Further advantages can also be achieved in that the partitions providedbetween the compartments can be heated. It is also advantageous if amultistage shaft sealing lip ring system operated with a buffer fluid isprovided for sealing the agitator shafts relative to the outeratmosphere. In order to seal the agitator shafts relative to the outeratmosphere, it is also possible to use a dual-action axial face sealsystem operated with a buffer fluid. If necessary, additional cooling ofthe sealing system can be integrated. These sealing systems allow amaintenance-free operation life of several years.

The special design characteristics of the disk-ring reactor according tothe invention are apparent from the enclosed FIG. 1 and FIG. 2.

LIST OF REFERENCE NUMERALS

-   1 rotating annular disk-   2 last partition-   3 outflow opening-   4 product outlet-   5 backflow openings-   6 agitator inlet side-   7 shaft wiper-   8 shaft-   9 hub-   10 spoke-   11 unheated separating plates.-   12 heated separating plates-   13 sector covered with perforated plate-   14 eccentricity-   15 baffle-   16 annular disks

1. A method for producing polyesters such as polyethylene; polypropyleneand polybutylene terephthalate from melts of precondensates ofpolyesters wherein vapors comprising precondensate componentsdistributed in the form of an aerosol are conducted through apolycondensation reactor in which precondensate components are depositedon the reactor wall and in an outlet chamber of the reactor on aseparator, the deposits are then conducted to the unstirred dischargesump, and upper layers of the discharge sump are continuouslyrecirculated in the stirred reactor area and thus subjected toreconversion and further polycondensation.
 2. The method according toclaim 1 wherein an intrinsic viscosity of the precondensate is limitedto a maximum of less than or equal to 0.37 dl/g, preferably to 0.21 to0.33 dl/g.
 3. The method according to claim 1 characterized wherein theprecondensate-containing vapors are conducted along the ceiling wall fora partial preliminary separation of precondensate as a film.
 4. Themethod according to claim 1 wherein peripheral movement of the agitatorelements of the polycondensation reactor on the product inlet side ishigher than on the product outlet side.
 5. The method according to claim1 wherein the outlet chamber has a stationary, controlled product level.6. The method according to claim 1 wherein the intrinsic productviscosity between IV is selected equal to between 0.5 dl/g and 1.35dl/g, preferably in the range between 0.55 dl/g and 1.25 dl/g.
 7. Themethod according to claim 1 wherein a disk-ring reactor is used as thepolycondensation reactor.
 8. A disk-ring reactor for producingpolycondensates according to the method from claim 1, comprising aheatable, cylindrical, horizontal vessel with at least one inlet and atleast one outlet for the melt, with a compartmenting of the productchamber by means of separating plates and an outflow opening in eachseparating plate as well as at least one vapor outlet provided above amelt outlet and an internal agitator system that is adapted to thevessel shape, which system comprises vertically disposed annular disksattached to at least one common horizontal drive shaft by means ofspokes wherein the last separating plate provided between the unstirredoutlet chamber and the compartment upstream from this chamber has atleast one outflow opening on the emersion side of the agitator as wellas at least one backflow opening on the immersion side of the agitator.9. The disk-ring reactor according to claim 8 wherein at least onebackflow opening of the last separating plate is configured as asteadily expanding, peripheral opening on the wall side, extending fromthe lowest vessel point.
 10. The disk-ring reactor according to claim 8wherein in addition to the wall-side, peripheral backflow opening anoverflow opening delimiting the discharge sump is provided radiallyinside the last annular disk.
 11. The disk-ring reactor according toclaim 8 wherein the agitator axis is positioned eccentrically relativeto the housing axis.
 12. The disk-ring reactor according to claim 8wherein upstream from the vapor outlet in the region of the outletchamber a separator, for example a baffle, a mist separator, a demisteror another suitable flow deflection device is provided to separate theentrained or aerosol-like dispersed product components.
 13. Thedisk-ring reactor according to claim 8 wherein a first agitator shaft isfollowed by a second agitator shaft rotating at reduced speed and withthe same or greater eccentricity of the shaft axis relative to thevessel axis in the outlet region.
 14. The disk-ring reactor according toclaim 8 wherein product-lubricated bearings are provided to support andinternally seat the agitator shafts.
 15. The disk-ring reactor accordingto claim 8 wherein the reactor comprises an inlet for the precondensateflow on one end and an outlet for the melt on the other end.
 16. Thedisk-ring reactor according to claim 8 wherein the precondensate flow isdivided into two partial flows, which are conducted entirely orpartially separate through the reactor.
 17. The disk-ring reactoraccording to claim 16 wherein the two partial flows are guided towardeach other through the reactor and are removed through a common productoutlet at the center from the reactor.
 18. The disk-ring reactoraccording to claim 16 wherein the two partial flows are conducted towardeach other to separate product outlets on the cover sides.
 19. Thedisk-ring reactor according to claim 8 wherein the annular disks areattached to a shaft by means of spokes and hubs.
 20. The disk-ringreactor according to claim 8 wherein the spokes of the annular disks aredisposed axially aligned.
 21. The disk-ring reactor according to claim 8to 19 wherein the spokes of adjoining annular disks are alternatelyoffset from each other by half a spoke sector angle between the spokesand disposed in groups axially aligned.
 22. The disk-ring reactoraccording to claim 8 wherein the spokes of consecutive annular disks aredisposed offset from each other in a screw-like manner.
 23. Thedisk-ring reactor according to claim 8 wherein disk ring agitatorshaving an even number of spokes alternately a normal free sector and asector covered with disk segments (metal plates, perforated plates)between the spokes and are disposed behind each other at least in pairssuch that free and covered sectors alternately overlap (in the axialperspective).
 24. The disk-ring reactor according to claim 8 wherein thepartitions provided between the compartments are heatable.
 25. Thedisk-ring reactor according to claim 8 wherein a multistage shaftsealing lip system operated with a buffer fluid is provided for sealingthe agitator shafts relative to the outer atmosphere.
 26. The disk-ringreactor according to claim 8 wherein a dual-action axial face sealsystem operated with a buffer fluid is provided for sealing theagitator-shafts relative to the outer atmosphere.